1. Field of the Invention
The present invention relates to a coating composition capable of curing to a high hardness by exposure to sunlight. More particularly, the coating composition comprises a high molecular weight polyurethane acrylate or methacrylate dissolved in one or more non-alcohol solvents, a visible light photoinitiator, and an alcohol solubilizer. Coatings derived from this composition have many desirable properties after evaporation of the solvents, namely a tack free surface, good hardness, good flexibility, and abrasion resistance. It has been found that the physical properties of the coating, particularly pencil hardness, are further enhanced with exposure to ambient sunlight. These sunlight cured coatings have superior hardness properties to coatings cured with traditional UV light curing equipment. The superiority of the sunlight cured coatings is mainly due to the coating forming a hard film after the solvent has evaporated. This film forms a barrier between the atmosphere and an acrylate or methacrylate in the system thus reducing the effect of oxygen inhibition. Without oxygen inhibition, the film can continue to crosslink and polymerize under low intensity light thereby improving the physical properties.
2. Description of the Related Art
There is great commercial interest in coatings which have good film forming capability, dry to be essentially tack-free when solvents are evaporated, and have a resulting high degree of pencil hardness after curing over time in sunlight. These coating compositions are different from other traditional coatings in several ways. These include simplicity, absence of a need to contain film formers such as nitrocellulose, cellulose acetate butyrate, or cellulose acetate propionate which are typically used in the art to produce solid films without light curing. Other materials, additives, or plasticizers that might be typically found in the art to improve flexibility are not required, although they may optionally be included. Also single coatings derived from the compositions have good hardness without being too brittle.